Self-aligned Contact (SAC) is one of the key technologies in ULSI (Ultra Large Scale Integration) fabrication. SAC can relax the alignment margin in photolithography. The selectivity to SiN at the corner (shoulder) is the most important issue in SAC etching.
Therefore, over the years of process development in SAC etching, heavy polymerization chemistries have been relied on in order to get high selectivity to nitride (i.e. ratio of [oxide etch rate]/[nitride etch rate]), because the nitride corner is exposed to plasma for a longer time or a longer over-etching than the bottom nitride. For achieving high corner selectivity, gases having high C/F atomic ratio, such as C4F8, are used in SAC etching. Additionally, CH3F or CH2F2 addition is reported to increase corner selectivity. Unfortunately, high selectivity SAC etching with these gases is also accompanied by the etch stop phenomenon, where the etching is incomplete, stopping in the middle of the hole due to fast polymerization, particularly in narrow slits between the gate electrodes. The width of the slit will be narrower with the shrinkage of the minimum feature sizes in LSIs. Thus it is becoming ever more difficult to keep the corner selectivity without encountering etch stop in the slit.
On the other hand, while the nitride corner is exposed, oxide in a narrow slit between the gate electrode is continuously being etched and oxygen is released which slows down the polymer formation on the nitride corner. It is also known that there is usually observed a lower selective portion on the small overlapped nitride corner and at the worst case, this phenomenon causes an electrical short problem.
The etch stop phenomenon is particularly a problem when the etching parameters must be controlled in a narrow process window. This requires very close control of etching parameters. On the other hand, the superior and strong carbon rich polymerization often cases various other problems such as an etching profile micro loading between dense and isolated patterns, a slow etching rate, low throughput and short mean time between wet cleanings (MTBWC).
Conventionally, it has been essential to separate island contact etching on a peripheral area or gate contact etching processes from SAC etching because it is very difficult to get a vertical profile and maintain enough good contact resistance when etched at the same time or with the same process due to superior and strong carbon rich polymerization.
One conventional method for SAC etching employs dipole ring magnetron reactive ion etching (DRMRIE). In DRMRIE, the plasma ion density is considered medium density and is from 1010-1011 ions/cm3. This provides a very stable, repeatable plasma with high selectivity. Unfortunately, the conventional use suffers from a low etching rate and tends to leave the corners from the etching process too sharp. Conventional etching uses a plasma generated using a pressure of from 50-60 mT and an RF power of 1300-1500 W.
Additionally, in conventional etching processes, the way to achieve higher selectivity is considered to be by increasing the bottom temperature (temperature at the bottom of the layer being etched) and to reduce the power. Unfortunately, these two procedures tend to cause etch stopping, described above.
Table 1 below provides a summary of various issues present in the SAC etching process conventionally practiced.
TABLE 1Table of various process issues/requirements of SAC etchingRequirements onRequirements onprocess integrationSAC etchingProcess Issuesaround SAC etchprocess(1) Current leakageTo minimize the mis-To control the etchbetween W-plug andalignment of the Inter-CD-bias on top ofInter-connect lineconnect linethe contact hole(2) Incomplete etchingto optimize the cellTo have enough bigdown to the Si substratelayout to have enoughetching capabilitycontact space(3) Electrical shortTo optimize the thicknessTo generate C-richbetween W-plug andand materials of the Gatepolymer to obtainPoly GateCap/Spacer, To controlhigh selectivity tothe overlap amount asthe Nitride or SiONmuch as possible by theLithography process(4) Small Bottom CD onN/ATo optimize thethe island contact holeetching chemistriesif it exists(5) Etching profileN/ATo optimize theU-loading between denseetching chemistriesand isolated areas
Accordingly, a process is needed for SAC etching that avoids the etch stop phenomenon, while maintaining high selectivity and high etching rate, with a large process window.